1. Field of the Invention
The present invention relates to a magnetic device, and particularly to a so-called release type magnetic device having improved attraction stability for use in camera shutters.
2. Description of the Prior Art
In a magnetic device used for controlling the camera shutter time and the like, a current is initially supplied to coils provided on the device's yoke, and when the current supply is interrupted, the armature is disengaged from the yoke by a return spring to cause disengagement of a member that has prevented the shutter's running movement.
In the above-mentioned release type magnetic device, the time period from interruption of the current supply to the coils, until the armature is disengaged from the yoke in order to operate other members, must be small and stable.
Generally, a magnetic device that frequently exercises magnetic attraction, for example, a device to control a shutter mechanism, requires a high durability attracting surface, so that a large attracting surface as a whole is needed. However, when the attracting surface is large, it is difficult for the armature to be disengaged from the yoke, even after interruption of the current supply. This is inconvenient.
Accordingly, in order to eliminate the above-mentioned inconvenience, it is desirable that an air gap should be formed in the state in which the yoke attracts the armature.
In order to obtain such an air gap in a stable manner, it is best to plate the magnetic metal. Further, with such a metal layer, a shock absorbing effect can be obtained at the time of attraction, which further contributes to durability.
However, with conventional copper plating, it is difficult to obtain sufficient thickness and durability which is inconvenient. Further, in order to increase the corrosion resistivity, it is necessary to plate with chrome and the like.